FIG. 5 is an exploded perspective view showing a schematic structure of a conventional measuring instrument for a vehicle which is disclosed in Patent Literature 1, FIG. 6 is a plan view showing a light guide member in the measuring instrument which is taken out therefrom, FIG. 7 is a sectional view as seen in a direction of arrow marks VII-VII in FIG. 6, and FIG. 8 is a sectional view as seen in a direction of arrow marks VIII-VIII in FIG. 6.
This measuring instrument for a vehicle is a combination meter in which a speed meter section, a fuel meter section, a water temperature meter section, a gear position section, etc. are integrated, and includes, as shown in FIG. 5, a body case 110, a light guide member 120 which is provided at a surface side of the body case 110, a dial plate 130 which is arranged at a surface side of this light guide member 120, and a pointer 140 which rotates on the surface of this dial plate 130. Moreover, this meter has a movement 150 which is arranged at a back face side of the body case 110 for rotating the pointer 140, a circuit board 160, a light source L which is mounted on the circuit board 160, and so on.
In the speed meter section of the dial plate 130, there are provided a scale design 131 which is formed in a circular arrangement along a rotation direction of the pointer 140, a warning design 133, and so on. These designs 131, 133 are so adapted as to be lit up with an illuminating light from the back face side.
The light guide member 120 mainly illuminates the scale design 131 of the dial plate 130. As shown in detail in FIG. 6, the light guide member 120 has a flat plate portion 121 in a shape of a semicircular disc corresponding to the speed meter section of the dial plate 130, a pointer shaft insertion hole 125 arranged at a center of the semicircular shape, light receiving portions 122 in a shape of conical wall arranged near the pointer shaft insertion hole 125, which will be described below, a through hole 123 provided corresponding to the warning design 133, three triangular holes 126 arranged between the pointer shaft insertion hole 125 and the light receiving portions 122, and light guide paths 127 interposed between two of the three triangular holes 126 thereby to guide the light which has been reflected by the light receiving portions 122 to a reflecting face 129 around the pointer shaft insertion hole 125. The reflecting face 129 around the pointer shaft insertion hole 125 performs a function of lighting up the pointer 140, by introducing an incident light to a light receptive face in a base part of the pointer 140, as its light path is shown in FIG. 8.
The light receiving portion 122 is a part for performing a function of receiving a light from a spot-like light source L (hereinafter referred to as a point light source=LED) which is arranged so as to be opposed to the light receiving face 122a at a back face side thereof, and radially diffusing the light toward the flat plate portion 121. As shown in FIG. 7, the light receiving portion 122 includes a projected part in a truncated conical shape which is projected toward the back face side of the flat plate portion 121, and a recess in a conical shape is concentrically formed inside the truncated conical shape to be dented from a surface side of the flat plate portion 121. In this manner, the light receiving face 122a is formed on a top face of the projected part in the truncated conical shape, and reflecting faces 122b, 122c, 122d, 122e are formed on conical faces outside and inside the projected part. In this prior art, an angle θ1 of an inside conical face and an angle θ2 of an outside conical face are set to be stepwise varied in such a manner that inclination angles of the reflecting faces 122b, 122c, 122d, 122e may approach to that of the flat face plate 121, as approaching to the flat plate portion 121.
Moreover, as shown in FIG. 5, the body case 110 is provided with a light guide member containing part 111 for containing the light guide member 120 at a surface side thereof. Dented parts 112 for containing the light receiving portions 122, a triangular tube part 113 to be fitted to the through hole 123 in the light guide member 120, and a pointer shaft hole 115 for passing the pointer shaft therethrough are formed in the light guide member containing part 111. Then, the back face of the light guide member 120 is contained in the light guide member containing part 111, the light receiving portions 122 of the light guide member 120 are contained in the dented parts 112, and the triangular tube part 113 to which a light of another point light source is introduced is fitted into the through hole 123 in the light guide member 120.
Because the measuring instrument is constructed as described above, the light R1 from the point light source L enters into the light guide member 120 from the light receiving face 122a of the light receiving portion 122, as shown in FIG. 8, and then, radially guided to the flat plate portion 121 sequentially repeating reflections on the reflecting faces 122b, 122c, 122d, 122e, whereby the scale design 131 of the dial plate 130 is illuminated from the back face side.